Perhaps the most widely used process for the manufacture of acetic acid, the well known Monsanto process, involves carbonylating methanol in the presence of rhodium, methyl iodide, methyl acetate and water. The product is suitable for all purposes; however, iodide contamination is an issue with respect to acetic acid made by way of the Monsanto process or in acetic anhydride manufactured by way of a rhodium-iodide catalyst system.
It was discovered by Hilton that macroreticulated, strong acid cationic exchange resins with at least one percent of their active sites converted to the silver or mercury form exhibited remarkable removal efficiency for iodide contaminants in acetic acid or other organic media. The amount of silver or mercury associated with the resin may be from as low as about one percent to as high as 100 percent. Preferably about 25 percent to about 75 percent of the active sites were converted to the silver or mercury form and most preferably about 50 percent. There is disclosed in U.S. Pat. no. 4,615,806 removal of various iodides from acetic acid. In particular there is shown in the examples removal of methyl iodide, HI, I.sub.2 and hexyl iodide.
Various embodiments of the basic invention disclosed in U. S. Pat. No. 4,615,806 have subsequently appeared in the literature. There is shown in U.S. Pat. No. 5,139,981 to Kurland a method for removing iodides from liquid carboxylic acid contaminated with a halide impurity by contacting the liquid halide contaminant acid with a silver (I) exchanged macroreticular resin. The halide reacts with the resin bound silver and is removed from the carboxylic acid stream. The invention in the '981 patent more particularly relates to an improved method for producing the silver exchanged macroreticular resins suitable for use in iodide removal from acetic acid.
U.S. Pat. no. 5,227,524 to Jones discloses a process for removing iodides using a particular silver-exchanged macroreticular strong acid ion exchange resin. The resin has from about 4 to about 12 percent cross-linking, a surface area in the proton exchanged form of less than 10 m2/g after drying from the water wet state and a surface area of greater than 10 m.sup.2 /g after drying from a wet state in which the water has been replaced by methanol. The resin has at least one percent of its active sites converted to the silver form and preferably from about 30 to about 70 percent of its active sites converted to the silver form.
U.S. Pat. No. 5,801,279 to Miura et al discloses a method of operating a silver exchanged macroreticular strong acid ion exchange resin bed for removing iodides from a Monsanto type acetic acid stream. The operating method involves operating the bed while elevating the temperatures in stages and contacting the acetic acid and/or acetic anhydride containing the iodide compounds with the resins. Exemplified in the patent is the removal of hexyl iodide from acetic acid at temperatures of from about 25.degree. C. to about 45.degree. C.
So also, other ion exchange resins have been used to remove iodide impurities from acetic acid and/or acetic anhydride. There is disclosed in U.S. Pat. No. 5,220,058 to Fish et al the use of ion exchange resins having metal exchanged thiol functional groups for removing iodide impurities from acetic acid and/or acetic anhydride. Typically, the thiol functionality of the ion exchange resin has been exchanged with silver, palladium, or mercury.
There is further disclosed in European Publication No. 0 685 445 A1 a process for removing iodide compounds from acetic acid. The process involves contacting an iodide containing acetic acid stream with a polyvinylpyridine at elevated temperatures to remove the iodides. Typically, the acetic acid was fed to the resin bed according to the '445 publication at a temperature of about 100.degree. C.
With ever increasing cost pressures and higher energy prices, there has been ever increasing motivation to simplify chemical manufacturing operations and particularly to reduce the number of manufacturing steps. In this regard, it is noted that in U.S. Pat. No. 5,416,237 to Aubigne et aL there is disclosed a single zone distillation process for making acetic acid. Such process modifications, while desirable in terms of energy costs, tend to place increasing demands on the purification train. In particular, fewer recycles and fewer purification steps tend to introduce (or fail to remove) a higher level of iodides into the product stream and particularly more iodides of a higher molecular weight. For example, octyl iodide, decyl iodide and dodeycl iodides may all be present in the product stream as well as hexadecyl iodide.
The prior art resin beds operated as described above do not efficiently and quantitatively remove higher organic iodides from organic media such as acetic acid or acetic acid streams as required by certain end uses, particularly the manufacture of vinyl acetate monomer. Accordingly, an object of the present invention is to provide for the efficient and nearly quantitative removal of higher organic iodides from an acetic acid and/or acetic anhydride product stream.